Unsaturated quaternary ammonium salts, such as acryloyloxyethyl trimethylammonium chloride (Cl−(CH3)3N+CH2CH2OC(O)CH═CH2; CAS Reg. No. 44992-01-0), are widely used as co-monomers in the synthesis of flocculant polymers for water treatment. The unsaturated quaternary ammonium salts are typically prepared by alkylation of the corresponding tertiary amine with an alkyl halide, such as methyl chloride. The literature includes many specific procedures for carrying out these reactions. In these procedures, methyl chloride is typically used in molar excess relative to the tertiary amine. See, for example, U.S. Pat. No. 4,520,210 to Schneider et al., U.S. Pat. No. 4,745,214 to Hess et al., U.S. Pat. No. 5,260,480 to Lacroix et al., U.S. Pat. No. 5,912,383 to Riondel, U.S. Pat. No. 6,521,782 to Riondel et al., U.S. Pat. No. 6,683,203 B2 to Druzkowski et al., U.S. Pat. No. 7,151,190 of Riondel, and U.S. Pat. No. 7,183,434 B2 to Baan et al.; Japanese Patent Application Publication Nos. JP 01-249749 A of Inagaki et al., JP 02-212457 A of Iwasaki et al., JP 06-032768 of Masuda et al.; European Patent Application Publication No. EP428970 A1 of Ascherl et al.; and International Patent Application Publication No. WO 00/43348 of Riondel et al. Reactions run in the presence of a stoichiometric excess of methyl chloride generally produce a high conversion of the tertiary amine and therefore largely avoid problems associated with separating the product unsaturated quaternary ammonium salt from residual tertiary amine reactant. However, such reactions have disadvantages that include requiring relatively high reaction pressures (to produce a high solution concentration of methyl chloride, which is a gas at ambient conditions), requiring special procedures and equipment for recycling excess methyl chloride, and requiring special procedures and equipment to maintain a substantial oxygen concentration while removing excess methyl chloride from the product solution (oxygen inhibits the undesired polymerization of the unsaturated quaternary ammonium salt). It would therefore be desirable to have a method of producing unsaturated quaternary ammonium salts that avoids difficult separations of the unsaturated quaternary ammonium salt from the unsaturated tertiary amine while reducing or eliminating the problems associated with handling excess methyl chloride.